(1) Field of the Invention
The present invention relates to a solid polymer electrolyte and to an electrochemical cell employing such an electrolyte. In a preferred aspect, the invention relates to a lithium-ion electrochemical cell employing a novel and improved solid polymer electrolyte/separator which can be processed as a liquid and to a method for assembling such a cell. The invention is particularly directed to an improved method for the facile fabrication of electrochemical cells with improved safety characteristics.
(2) Description of the Prior Art
Electrochemical cells, such as Li-ion cells, require an electrolyte to permit transport of the cationic species, in this example, lithium ions, between the cathode and anode as the cell is charged and discharged. To prevent electronic short circuit in the cell, an electrically insulating material is need to separate the cathode and anode in the cell.
The prior art includes a wide variety of mixtures which may be classified as liquid electrolytes. When applied to lithium-ion electrochemical cells, prior art electrolytes include those consisting of a solution of a salt, such as lithium hexaflurorophosphate or lithium hexafluoroborate, in a solvent mixture typically containing one or more carbonate type solvents, such as propylene carbonate, ethylene carbonate or dimethyl carbonate. Liquid electrolytes have a number of significant disadvantages, notably, they freeze at low temperatures, are liquid and free flowing at intermediate temperatures and thus able to leak and they degrade at elevated temperatures. Moreover, liquid electrolytes have a high volatility and are thus able to vent and burn.
The prior art also includes polymer electrolytes. These electrolytes are solid phase organic materials that have appreciable ionic conductivity, typical for lithium. They include polyethylene oxide, polyether and poly(dimethylsiloxane) polymers, for example. The ionic conductivity of these polymer electrolytes is usually poor at room temperature and accordingly they must be heated to above about 40.degree. C. to operate. Unfortunately, none of these electrolytes operate at low temperatures. Electrochemical cells or batteries which utilize these electrolytes are typically maintained at temperatures between about 40.degree. C. and 80.degree. C. during operation. Below40.degree. C. the ionic conductivity of the electrolyte is too low for operation of the cell whereas at higher temperatures, above about 80.degree. C., decomposition of the organic polymer ensues. Moreover, for processing reasons, the minimum useable thickness of some polymer electrolytes is over double that typical in a liquid electrolyte cell. The increased cathode-anode distance and lower ionic conductivity result in poor rate capability in such polymer electrolyte cells when compared to liquid electrolyte cells.
Gel electrolytes are another class of electrolytes known in the prior art. They typically embody a polymer host, such as poly(ethylene oxide) or poly(vinylidine fluoride) or copolymers such as poly(vinylidine fluoride)-hexafluoropropylene and a liquid electrolyte guest or "plasticizing agent" containing a lithium salt and a solvent such as ethylene carbonate or dimethylcarbonate. Gel electrolytes are usually fabricated as a free standing film. Cells utilizing gel electrolytes must be fabricated by incorporating the polymer host between the cathode and anode during cell fabrication. Such materials are typically "activated" after cell fabrication by addition of liquid electrolyte which is absorbed into the polymer host material.
Gel electrolytes possess all of the disadvantages of liquid electrolytes with many of the inhibitions of polymer electrolytes. They achieve a conductivity which is close to but below that of liquid electrolytes by incorporating liquid electrolytes into a polymer host. As a result, they have volatility and flammability comparable to liquid electrolytes. Gel electrolytes also have processing inhibitions similar to polymer electrolytes. As a result, cells with gel electrolytes typically have rate capability inferior to comparable cells which use a liquid electrolyte.
Solid electrolytes are also known in the prior art. They include ceramic materials such as lithium phosphorous oxynitride and are able to conduct lithium ions. Ceramic materials typically have ionic conductivity 1000 times less than liquid electrolytes and are brittle. Such materials must be deposited by a plasma or gas phase method such as sputtering onto one electrode and then the other electrode deposited onto the electrolyte layer. To date, solid electrolytes have only found application in very small cells due to manufacturing limitations inherent in a ceramic device. Solid electrolytes present many difficulties in achieving a viable interface between the electrolyte and the electrode materials.
In a recent article entitled "Synthesis and Properties of Sol-Gel Derived Electrodes and Electrolyte Materials", by J. Harreld et al appearing in "The Proceedings of the 5th Workshop for Battery Exploratory Development", published on Jun. 30, 1997, there is disclosed a solid electrolyte material which exhibits a high lithium, ion conductivity. The solid electrolyte was prepared by a known sol-gel process wherein a hydrolyzed silica precursor, namely, (tetramethyl) orthosilicate, Si(OCH.sub.3).sub.4, was admixed with a lithium, ion conducting liquid electrolyte along with deionized water and an acid catalyst to form a lithium conductive sol. The liquid lithium electrolyte was prepared by dissolving ethylene carbonate with lithium borofluorate, LiBF.sub.4 in a propylene carbonate solvent to a molarity of 1.65M. After ageing and drying, Si--O--Si linkages form within the sol and a three-dimensional silicate network develops in which the liquid phase is encapsulated. The liquid electrolyte provides ionic conductivity while the silica linkages support the liquid electrolyte.
Experimentation with the solid electrolyte material disclosed in the above article has shown that the lithium borofluorate, LiBF.sub.4, component in the liquid electrolyte reacts with water in the reaction mixture and is not stable. This of course precludes use of this solid electrolyte material in the fabrication of a working electrochemical cell.
Reference is also made to an article entitled "Sol-Gel Approaches for Solid Electrolytes and Electrode Materials" by B. Dunn et al, appearing in "Solid State Ionics", (1994). This article describes the sol-gel process in greater detail and the concept-of using an inorganic gel in combination with an organic ionic conductor. The authors describe the use of only one salt in preparation of the liquid electrolyte, namely, lithium perchlorate, LiClO.sub.4. They also note that in addition to silicon alkoxides, other metal alkoxides could be used in the sol-gel process such as the metal alkoxides of aluminum, titanium, vanadium, molybdenum and tungsten.